Highly efficient and chemoselective transfer hydrogenation of nitroarenes at room temperature over magnetically separable Fe-Ni bimetallic nanoparticles

Article abstract of DOI:10.1039/c3ra45787g

A highly chemoselective catalytic transfer hydrogenation (CTH) of nitroarenes to corresponding amino derivatives is achieved with Fe-Ni bimetallic nanoparticles (Fe-Ni NP's) as the catalyst and NaBH₄ at room temperature. Their catalytic efficiency is ascribed to the presence of Ni sites on the bimetallic surface that not only hinder the surface corrosion of the iron sites but also facilitate efficient electron flow from the catalyst surface to the adsorbed nitro compounds. This facet is corroborated with reusability studies as well as surface characterization of the catalyst before and after its repetitive usage. Thus, these nanoparticles efficiently catalyze the reduction of functionalized nitroarenes to corresponding amines without use of corrosive agents like base or other additives under ambient conditions and are easily separated by a laboratory magnet in an eco-friendly manner.

Full text of DOI:10.1039/c3ra45787g

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Highly efficient and chemoselective transfer
hydrogenation of nitroarenes at room temperature
over magnetically separable Fe–Ni bimetallic
nanoparticles†
Cite this: RSC Adv., 2014, 4, 8004
*
Dhananjay R. Petkar, Brijesh S. Kadu and Rajeev C. Chikate
A highly chemoselective catalytic transfer hydrogenation (CTH) of nitroarenes to corresponding amino
derivatives is achieved with Fe–Ni bimetallic nanoparticles (Fe–Ni NP's) as the catalyst and NaBH4 at
room temperature. Their catalytic efficiency is ascribed to the presence of Ni sites on the bimetallic
surface that not only hinder the surface corrosion of the iron sites but also facilitate efficient electron
flow from the catalyst surface to the adsorbed nitro compounds. This facet is corroborated with
reusability studies as well as surface characterization of the catalyst before and after its repetitive usage.
Thus, these nanoparticles efficiently catalyze the reduction of functionalized nitroarenes to
corresponding amines without use of corrosive agents like base or other additives under ambient
conditions and are easily separated by a laboratory magnet in an eco-friendly manner.
Received 12th October 2013
Accepted 25th November 2013
DOI: 10.1039/c3ra45787g
www.rsc.org/advances
In order to overcome these lacunas, an alternative approach is
adopted wherein various hydrogen donors like hydrazine,
1. Introduction
ammonium formate, IPA and NaBH4 are used for liquid phase
catalytic transfer hydrogenation (CTH) of functional aromatics.
This approach is considered to be safe, cost-effective, selective
and eco-friendly. Employing this strategy, a highly chemo-
selective reduction of aromatic nitro compounds to the corre-
sponding amino derivatives is achieved by a combination of
copper nanoparticles and ammonium formate10 in ethylene
Selective catalytic hydrogenation of functional nitroarenes to
anilines is an industrially important process for synthesis of
agrochemicals, pharmaceuticals, dyestuffs, urethanes and
other industrially important products.1–3 Conventionally, this
reduction is carried out with Fe/HCl that requires a stoichio-
metric excess of reagents which eventually produces a large
amount of secondary waste and therefore, is not considered an
environmentally benign process. RANEY® nickel is widely used
as the catalyst but has two major disadvantages: moisture
sensitivity and a pyrophoric nature. Pd/C is also known to
catalyze this reaction but it is more expensive and also quite
sensitive to trace impurities. Besides Pd, a number of other
transition metal catalysts like Pt, Ru are also reported to be
efficient for this transformation. However, these methodologies
require a cumbersome experimental set up like an autoclave
and the process is carried out under high temperature–pressure
conditions in the presence of a catalyst and H2 gas.4,5
Concomitantly, this conversion is also reported to occur at room
temperature with H2 gas with a variety of catalysts6 as well as for
other types of reactions such as alkenes, alkynes, olens,
ketones and cyclohexene with 90–100% conversion.7–9
ꢀ
glycol at 120 C. On the other hand, selectivity for a Rh nano-
catalyst is demonstrated for the reduction of nitroarenes with
hydrazine1 monohydrate at 60 C while a similar potentiality is
ꢀ
exhibited by Ru0-nanoparticles for the transfer hydrogenation of
substituted nitrobenzenes in IPA with retention of its activity for
several cycles.11 A facile, simple and environment friendly
hydrogen-transfer reaction over recyclable ferrite–nickel
magnetic nanoparticles (Fe3O4–Ni) using glycerol as the
hydrogen source affords the synthesis of aromatic amines and
alcohols from nitroarenes and carbonyl compounds.12 Further-
more, ionic liquid supported Ni nanobers are found to be highly
efficient for reduction of substituted nitroarenes with NaBH4 at
room temperature with >90% conversion within 2 h.13 On the
other hand, oxide based heterogeneous catalysts have also been
explored for CTH. For example, CeO2 nanorods exhibited excel-
lent catalytic behaviour with high conversion and selectivity for a
variety of aromatic nitro compounds using N2H4.14 Reduction of
nitro derivatives to amines using IPA as a hydrogen source and
KOH as a promoter is reported for a nano-catalyst MgO–ZrO2.15
Solid supported nano- and microparticles of Pd successfully
convert nitroarenes to amines with various hydrogen donors.16
Nanoscience Group, Department of Chemistry, Post-graduate and Research Centre,
MES Abasaheb Garware College, Karve Road, Pune-411004, India. E-mail:
rajuchikate29@gmail.com; Fax: +91-20-25438165; Tel: +91-20-41038263
† Electronic supplementary information (ESI) available: TEM image of Fe–Ni NP's,
catalyst amount optimization, GC-MS data of CTH reactions and catalyst activity
table. See DOI: 10.1039/c3ra45787g
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With increasing environmental concerns, much interest has 13 mmoles of NaBH4 dissolved in 5 mL of methanol is added
been generated towards facile, sustainable and eco-friendly drop wise with constant stirring till addition of NaBH4 is
processes that include easily accessible raw materials, and complete. The whole reaction mixture is further stirred for the
simplicity in designing the catalysts that are efficient as well as desired period of time during which the progress of the reaction
inexpensive. To address this issue, we have selected iron based is monitored with TLC. Aer completion of the reaction, the
catalysts because of their low cost, nontoxicity and environ- catalyst is separated magnetically as well as by centrifugation
mentally benign properties. The potentiality of iron based (3000 rpm) and the separated reaction solution is subjected to
catalysts, either in bulk or nano-form, has been explored for GC-MS analysis. To investigate the reusability of Fe–Ni NP's, the
reductive degradation of aquatic contaminants; however, their magnetically separated catalyst is washed successively with
repeated use is restricted due to surface corrosion. By virtue of acetone and vacuum-dried at room temperature before further
forming bimetallic species with other noble or non-noble catalytic cycles.
metals, it is observed that such bimetallic systems remain active
over longer periods due to the presence of other metal centers
which eventually lead to increased catalytic efficiency and life-
3. Results and discussion
3.1. Characterization of Fe–Ni NP's
span of iron based catalysts. Keeping this view in mind, we have
synthesized Fe–Ni NP's and employed them for transfer
hydrogenation of nitroarenes with NaBH4 as the hydrogen
source owing to its non-ammability, easily hydrolysable nature
and controlled H2 production rate.17
The XRD pattern of the Fe–Ni NP's (Fig. 1a) exhibits character-
istic peaks corresponding to Fe–Ni alloy with diffraction
patterns in the range of 40–65ꢀ that are assigned to (111) and
(200) crystal planes of fcc (JCPDS 47-1417) as well as bcc (JCPDS
37-0474) Fe–Ni alloy.19 Additionally, the XRD peak at 40ꢀ can be
ascribed to face centred cubic (fcc) of Fe–Ni alloy, which is
consistent with the standard card (JCPDS card no. 38-0419). The
morphology of Fe–Ni NP's (ESI; Fig. S1†) is found to be a
2. Experimental
2.1. Chemicals
The following analytical grade chemicals were purchased from
Loba Chemie, UK and used as received: iron(II) sulphate (FeS-
O4$7H2O), nickel(II) sulphate (NiSO4$6H2O), sodium borohy-
dride (NaBH4). Deionized water is used throughout the
experiments. All glassware and Teon-coated magnetic stir bars
are cleaned with acetone, followed by copious rinsing with
spherical shape (20–40 nm) with surface area of 14.2 m2 gꢁ1
.
3.2. Catalytic activity
It has been reported earlier that nitrobenzene is reduced by
iron under anaerobic conditions to aniline in aqueous
medium with nitrosobenzene as an intermediate product;
however, a decreased reduction rate is attributed to the
precipitation of siderite on the iron surface that inhibits
further reduction processes.20 Although, hydrogenation of
nitroarenes by nickel nanoparticles and hydrogen gas is
ꢂ100%, such an approach is of limited use as these reactions
are carried out at elevated temperatures (100 ꢀC) that results in
oxidation of the nickel surface.21 Moreover, nickel nano-
particles with isopropanol are proved to be suitable for che-
moselective hydrogenation of ketones at 76 ꢀC, however,
formation of nickel oxide lm on the surface of the catalyst
consequently hampers its catalytic activity.22 Considering
these shortcomings, we have adopted a rational approach
wherein the corrosion of Fe0 or Ni0 is controlled through the
formation of the bimetallic state that offers synergism on the
surface and subsequently may enhance the reduction capa-
bilities of the individual metals. In the present work, Fe–Ni
NP's are employed as a heterogeneous catalyst in transfer
hydrogenation of some substituted nitrobenzenes to the cor-
responding anilines (Table 1). Although there are several
reports on the transfer hydrogenation of nitroarenes, these
reactions are carried out at elevated temperatures as well as
requiring higher reaction times either with noble or non-noble
ꢀ
distilled water before drying in an oven at 100 C.
2.2. Synthesis of Fe–Ni NP's
Fe–Ni NP's are synthesized in accordance with the procedure
reported earlier.18
2.3. Characterization of nanocomposite
X-ray powder diffraction (XRD) patterns are recorded on a
Phillips X'pert MPD X-ray diffractometer using Cu-Ka radiation.
Transmission electron microscopic (TEM) images are obtained
using a JEOL electron microscope (model 1200X) while X-ray
photoelectron spectroscopy (XPS) analysis is carried out on a VG
Micro Tech ESCA3000 instrument using Mg-Ka radiation
(photo energy 1253.6 eV). Brunauer–Emmett–Teller (BET)
surface area analysis is performed using the nitrogen adsorp-
tion method with surface analyzer system CHEMBET3000,
Quantichrome Instruments, US. Kinetics of the transfer
hydrogenation are monitored with a UV-Vis spectrometer
(Lamda 25, Perkin Elmer) while the product analysis is carried
out with GC-MS (Clarus 500, Perkin Elmer).
2.4. Catalytic transfer hydrogenation
General method for the catalytic transfer hydrogenation of metals as heterogeneous catalysts.1,10 However, we have
nitroarenes is as follows: in a 50 mL three neck round bottom adopted a simple, facile and affordable route wherein Fe0 acts
ask, 1.6 mmoles of nitroarene is dissolved in 10 mL of meth- as an electron donor while Ni serves the purpose of electron
anol followed by addition of 0.16 mmoles of Fe–Ni NP's. The shuttle thereby forming a galvanic cell on the bimetallic
entire mixture is stirred at room temperature for 5 min and then surface. Such a feature of the Fe–Ni bimetallic system has
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Fig. 1 XRD of Fe–Ni NP's (a) fresh and (b) recovered catalyst after 6th cycle.
already been established for reductive degradation of organic reduced and not the ester function. The plausible reason for
and inorganic contaminants.18,23
such a facile conversion stems from the fact that the formation
To optimise the amount of Fe–Ni NP's required for CTH with of metal hydride on the catalytic surface is facilitated by
NaBH4, the nitrobenzene reaction is carried out at different expulsion of hydrogen generated by borohydride in the reaction
catalyst loadings (ESI; Fig. S2†). It is observed that conversion is medium. Concurrently, the adsorption of nitroarene on the Fe–
almost complete with 10 mol% which, therefore, is kept Ni NP's surface and simultaneous removal of a water molecule
constant throughout the investigation. Table 1 illustrates the lead to formation of the nitroso compound, which subsequently
transfer hydrogenation of nitroarenes in the presence of Fe–Ni combines with another molecule of hydrogen giving rise to
NP's carried out at room temperature with NaBH4 as the hydroxylamine. Finally, hydroxylamine is reduced to the amine
hydrogen source. It can be seen from this table that nitroben- derivative in a slow reaction step with water as a byproduct
zene (entry 1) is completely converted to aniline (ꢂ100%) within (Scheme 1). It is also worth mentioning here that the turnover
0.5 h with 100% selectivity (ESI; Fig. S3†). Interestingly, when frequency (TOF) is 13 times higher for Fe-Ni NP's as compared
either Fe0 or Ni0 nanoparticles are used for nitrobenzene to Pt–Ni bimetallic nanoparticles24 used for the transfer hydro-
reduction under identical conditions, only 50% conversion is genation of nitroarenes under identical conditions. Thus, such
observed with the former while reaction does not proceed using a faster reduction process and considerably higher TOF is
the latter catalyst (ESI Table T1†). To further access the rate of probably due to the efficient ow of electrons from Fe0 to the
conversion, we have also established the kinetics of the hydro- adjacent Ni0 site on the catalytic surface as well as the syner-
genation of nitrobenzene catalyzed by Fe–Ni NP's in the pres- gistic effect of both Fe–Ni and NaBH4 in tandem that plausibly
ence of NaBH4 (Fig. 2a). It can be seen from this gure that enhances the liquid phase reduction process at room
reduction of nitrobenzene proceeds initially with a rapid temperature.
decrease of reactant concentration up to about 97% but later on
it follows almost rst order reaction kinetics suggesting that the
reaction is almost complete within 35 min. These ndings
3.3. Reusability studies
clearly suggest that the catalytic activity is indeed due to both Fe The efficiency of Fe–Ni NP's is further evaluated for repeated
and Ni nanoparticles in a bimetallic phase through synergistic use without sacricing the catalytic activity. For this purpose,
effects on the catalytic surface.
transfer hydrogenation of nitrobenzene is carried out with
recovered catalyst under identical experimental conditions.
The conversion of nitroarenes to corresponding amines is
completed within a short span of time (0.5–6 h) with almost These results (Fig. 3) suggest that the catalyst remains highly
98% conversion and about 95% selectivity. The lower selectiv- active up to six cycles with >99% conversion for every cycle.
ities of entries 2 and 3 are probably due to the presence of bulky Moreover, the reaction rate remains almost constant for the
methyl while m-dinitrobenzene (entry 6) is converted to the 1st cycle (2.31 ꢃ 10ꢁ3 sꢁ1) up to the 6th cycle (2.25 ꢃ 10ꢁ3 sꢁ1).
mono-amine with low yield (80%) and lower selectivity (Table 1). Such a behavior can be explained on the basis of several
The latter conversion may be attributed to the strong desorption aspects: (i) methanol (solvent) induces faster methanolysis of
NaBH4 leading to rapid evolution of hydrogen25 that main-
tains nanoparticles well dispersed/suspended throughout
tendency of the formed basic m-nitroaniline on the negatively
charged catalytic surface. On the other hand, selectivity
for nitro-reduction is found to be ꢂ100% with almost 99% the reduction, (ii) a small amount of liberated H2 expels air
conversion for entries 7–9 implying that the catalyst possesses from the solution thereby preventing the surface corrosion of
high selectivity towards the nitro-function. A similar facet is the catalyst, (iii) some amount of the liberated hydrogen also
also observed for entry 10 where the double bond is selectively helps in retaining the zero valent state of the nanoparticles,
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Table 1 Catalytic transfer hydrogenation of nitroarenes to corresponding anilines
%
%
Entry
1
Substrate
Product
Time
Conversion
Selectivity
TOFa ꢃ 10ꢁ4 (sꢁ1
)
35 min
99.52
88.02
100
92
47.62
2
3
4 h
5 h
6.94
5.56
90.17
95.57
94
98
4
5 h
5.56
5
6
7
3 h
6 h
6 h
99.37
80.26
99.79
100
95
9.26
4.63
4.63
100
8
9
4 h
98.85
100
6.94
1 h
99.57
99.27
100
100
2.78
2.31
10
1.2 h
a
TOF ¼ TON/reaction time.
(iv) formation of NaB(OCH3)4 during the methanolysis of reacts with NaBH4 which further enhances hydrogen gener-
NaBH4 facilitates continuous evolution of hydrogen and (v) ation. Moreover, to ascertain that the rate of conversion is not
sodium methoxide formed during the transfer hydrogenation affected by successive usage of Fe–Ni NP's, we have
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Fig. 3 Reusability of Fe–Ni NP's for chemoselective reduction of
nitrobenzene.
Fig. 2 Time profile of nitrobenzene reduction for (a) fresh and (b)
recovered Fe–Ni NP's. Inset shows magnetic separation of catalyst
after completion of the reaction.
nitroarenes with high selectivity under simple laboratory
conditions.
These facts are further corroborated by the evaluation of the
catalyst surface properties of Fe–Ni NP's before and aer the
transfer hydrogenation reduction of nitrobenzene. Techniques
employed for this assessment include (i) XRD that assists in
differentiating crystalline phases of Fe–Ni NP's and (ii) surface
analysis by XPS which identies the nature of the chemical
species present on the surface of the Fe–Ni NP's before and aer
CTH. XRD analysis of these nanoparticles aer the 6th cycle
(Fig. 1b) indicates that there is no appreciable change in the
pattern although the intensities of peaks are somewhat
reduced. It is probably due to the successive exposure of these
planes to the chemical environment that may lead to slight
disturbances in the lattice structure. On the other hand, XPS
analysis of nanoparticles aer the last cycle was carried out for
the iron and oxygen regions (Fig. 4) to examine the extent of
surface corrosion that occurred on the nanoparticle surface. It
can be seen that the nature and peak positions at 710, 712.4 and
715.3 eV for the iron region (Fig. 4a) remain almost identical
with a slight change in their intensity pattern. Surprisingly, the
amount of Fe(II) formed on the surface has increased by about
7% aer the 6th cycle implying that some part of the liberated
hydrogen from NaBH4 has also been utilized for the reduction
of surface Fe(III) species. Such a surface phenomenon observed
for Fe–Ni NP's, therefore, suggests that the surface corrosion is
appreciably inhibited due to the presence of a reducing atmo-
sphere during the entire course of the reaction. Moreover, these
characteristics are also reected in the oxygen region where the
peaks at 530, 531.4 and 532.6 eV remain unchanged (Fig. 4b)
even aer the successive use of Fe–Ni NP's without much
alteration in their intensities. Thus, it may be argued that
almost 99% conversion of nitrobenzene to aniline with 100%
selectivity is predominantly due to the existence of an anaerobic
environment throughout six cycles and this approach may be
regarded as a necessary feature for sustainable development in
Scheme 1 Proposed mechanism for CTH of nitroarenes by Fe–Ni
NP's.
established the kinetics of the hydrogenation of nitroben-
zene catalyzed by recovered catalyst (Fig. 2b). It is interesting
to note that catalytic activity remains almost the same even
aer the 6th cycle which suggests that its surface is not
passivated even aer exposure to several chemical treat-
ments. Thus, all these factors contribute towards prevention
of surface corrosion of the nanoparticle surface that conse-
quently enhances its catalytic life. Such a feature has already
been observed in the degradation of a dye by Fe–Ni NP's
stored for 1 year; an orange G wherein it is observed that the
reduction capacity is retained even aer exposure to light and
moisture.26 Hence, it can be argued that this bimetallic
system is also efficient towards transfer hydrogenation of
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Fig. 4 XPS spectra of Fe–Ni NP's for (a) iron region and (b) oxygen region for fresh and recovered catalyst.
the eld of catalytic transfer hydrogenation of organics without rates and signicantly enhance the catalytic activity of Fe–Ni
sacricing the catalytic efficiency.
NP's.
3.4. Proposed mechanism
4. Conclusions
It is well known that metallic particles, either monometallic or
multimetallic, being electronically conducting solids, strongly The present investigation clearly demonstrates that nano-
catalyze redox reactions. The proposed mechanism for the engineered Fe–Ni NP's indeed exhibit promising catalytic
reduction of nitroarenes by borohydride in the presence of Fe– activity for transfer hydrogenation of nitroarenes under
Ni NP's is illustrated in Scheme 1. The catalytic hydrogenation ambient conditions with almost 99% conversion having 100%
promoted by a H-donor proceeds on the surface of the metal selectivity. Since the iron surface is susceptible to corrosion,
nanoparticles. The generation of H2 on the catalyst surface is it may be argued that such high efficiency of the catalyst may
due to efficient methanolysis of sodium borohydride25 forming be ascribed to synergism in Fe–Ni NP's that not only controls
metal hydrides followed by intermolecular hydrogen transfer the iron passivation but also facilitates efficient ow of electron
from the nanoparticle surface to adsorbed nitroarenes on the transfer from iron (donor) to nitroarenes (acceptor) mediated
catalyst surface. Subsequently, loss of a water molecule leads to by Ni site that acts as an electron shuttle between donor and
the formation of the nitroso compound, which being highly acceptor. This facet is further exemplied by their reusability
reactive, combines with another molecule of hydrogen giving potential where the catalyst surface remains active up to six
rise to stable hydroxylamine. Since the conversion of hydroxyl- cycles. To corroborate this aspect, XRD analysis of fresh and
amine to aniline through dehydroxylation is the slower and rate used catalysts suggests that the lattice structure remains almost
determining step,3 variation in the conversion time is noted the same even aer repeated use. Moreover, surface properties
from 0.5–6
h
for different nitroarenes. Water being are also not altered during the recycling process implying that
the byproduct of CTH, it is possible that the iron surface may the iron-corrosion process is effectively hindered due to adja-
become corroded due to adsorbed water. However, in our case, cent Ni sites. It is also quite likely that some of the liberated H2
the corrosion of the iron site is inhibited due to the presence of during CTH may be consumed by the catalyst to retain its
adjacent nickel sites as well as the anaerobic environment that activity. This catalytic system not only efficiently catalyzes the
prevents its surface passivation. The Ni sites also promote effi- reduction of functionalized nitroarenes to the corresponding
cient ow of electrons and subsequently increase reduction substituted anilines but also avoids the need for an inert
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atmosphere, and additional base or other additives. Thus, Fe– 12 M. Gawande, A. Rathi, P. Branco, I. Nogueira, A. Velhinho,
Ni NP's employed in the present investigation not only possess
excellent catalytic activity with very high chemo-selectivity
towards selective reduction of nitroarenes but they also can be
magnetically separated from the reaction medium.
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